1. Field of the Invention
The present invention relates to the treatment of liquid wastes which contain heavy metals.
2. Description of the Prior Art
Resource Conservation Recovery Act (RCRA) regulations require that wastes containing lead, chromium, cadmium, arsenic, mercury, selenium, silver or barium leach minimum concentrations of these metals to be legally defined as non-hazardous. The Environmental Protection Agency (EPA) has specified a standard test, the Extraction Procedure (EP) toxicity test, to determine compliance with the leachable metal concentration allowances. This test is described in 40 C.F.R. .sctn.261, Appendix II. Wastes that fail the EP toxicity test are hazardous and must be disposed of in permitted RCRA facilities. This can be difficult and expensive. Therefore, a process that removes and retards metals leachability ensuring that hazardous wastes are rendered non-hazardous would be a benefit to current waste management options.
Existing processes to reduce chemical mobility in wastes are mostly encapsulation processes, whereby various encapsulants are added to the waste, (i.e., liquids or semi-solids) to turn the waste into a solid. Examples of such encapsulants are cement and sodium silicate, asphalt, glass, and various polymers and chemicals. Emphasis is placed on creating a hard solid mass which water cannot penetrate. Encapsulation processes are used as final wastewater treatment steps to render sludges and liquids non-leachable. Unfortunately, encapsulation processes tend to be expensive and often greatly increase the amount of material for disposal, and may be subject to leaching if the encapsulation seal later fails.
Moreover, traditional heavy metals bearing wastewater treatments use lime or NaOH to precipitate the metals. Thus, metal hydroxides are precipitated, which hydroxides have a comparatively high solubility. Certain of the hydroxides are also amphoteric, i.e., soluble at low and high pH. Generally, the pH of treatment solutions must be maintained between about 6 and 10 to achieve maximum metal hydroxide insolubility. Furthermore, even after precipitation, the resulting sludge may not pass the EP toxicity test.
Therefore, a need exists for a process for treating liquid waste which contains heavy metals, which process results in the production of a non-hazardous sludge which can be disposed of in a landfill, and a liquid with the heavy metals content having been reduced to levels acceptable for discharge into existing sewer systems.